Miniaturized virtual grounding nonvolatile semiconductor memory device and manufacturing method thereof

ABSTRACT

The object of the present invention is to provide a new nonvolatile semiconductor memory device and its manufacturing method for the purpose of miniaturizing a virtual grounding type memory cell based on a three-layer polysilicon gate, enhancing the performance, and boosting the yield. In a memory cell according to the present invention, a floating gate&#39;s two end faces perpendicular to a word line and channel are partly placed over the top of a third gate via a dielectric film. The present invention can reduce the memory cell area of a nonvolatile semiconductor memory device, increase the operating speed, and enhances the yield.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 10/718,563filed Nov. 24, 2003 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nonvolatile semiconductor memorydevice and its manufacturing method, and more particularly to a methodfor enhancing the performance, increasing the degree of integration, andimproving the yield.

2. Description of the Related Art

Flash memories excel in portability and shock resistance. Further, theycan be electrically erased at a time. In recent years, therefore, theyhave been increasingly demanded as a filing device for use with mobilepersonal computers, digital still cameras, video cameras, and othersmall-size, portable information devices. Reducing the bit cost bydecreasing the memory cell region and enhancing the programming speedfor supporting a quick content download are essential for flash memorymarket expansion. Some memory cell methods for achieving such a purposeare now proposed. For example, a virtual grounding type memory cellbased on a three-layer polysilicon gate is disclosed (refer to JP-A No.028428/2001 or JP-A No. 085541/2001). As shown in FIG. 15, this memorycell comprises a floating gate 204, which serves as a first gate, acontrol gate 205, which serves as a second gate, and a third gate 206.The memory cell comprises a well 202 in a silicon substrate 201, asource in the well, drain diffusion layer regions 203, 203′, and a firstgate composed of polysilicon film formed on the well. The third gateincorporates the functionality of either an erase gate or a splitchannel control gate. Dielectric films 207, 208, 209, 210, 211 are usedto separate polysilicon gates 204, 205, 206 and separate the well 202from the polysilicon gates. The control gate 205 is connected in the rowdirection to constitute a word line. The source and drain diffusionlayer regions 203, 203′ are of a virtual grounding type, which shares anadjacent memory cell diffusion layer. This reduces the pitch in the rowdirection. The third gate 206 is perpendicular to a channel andpositioned perpendicularly to the word line 205. When a programmingoperation is to be performed, an independent positive voltage is appliedto the word line 205, drain 203, and third gate 206, and the well 202and source 203′ are set at 0 V. Hot electrons are then generated on achannel at a boundary between the third gate and floating gate andintroduced into the floating gate 204. This results in an increase inthe memory cell threshold voltage. When an erasing operation is to beperformed, a positive voltage is applied to the third gate 206, and anegative voltage is applied to the word line 205. The source 203′, drain203, and well 202 are set at 0 V. Electrons are then emitted from thefloating gate 204 to the third gate 206 so that the threshold voltagedecreases. A negative voltage is applied to the word line 205. The thirdgate 206, source 203′, drain 203, and well 202 are set at 0 V. Electronsare then emitted from the floating gate 204 to the well 202 so that thethreshold voltage decreases. The memory cell transistor thresholdvoltage is varied in this manner to distinguish between the informationvalues “0” and “1”.

However, new tasks arise when an attempt is made to increase thecapacity of the above nonvolatile semiconductor memory device.

The first task is to decrease the internal operating voltage forprogramming or erasing a memory cell. This task is particularly evidentwhen an attempt is made to erase a memory cell. As regards a flashmemory, the following equation generally expresses the relationshipbetween the control gate voltage Vcg and floating gate Vfg:Vfg=Vcg×C2/(C1+C2+Cag+Cfg)  Equation (1)

-   -   where C1 is the capacity of dielectric film (tunnel dielectric        film) between the floating gate and the Si substrate, C2 is the        capacity of dielectric film (interpoly dielectrics) between the        floating gate and the control gate, Cag is the capacity of        dielectric film between the floating gate and the third gate,        and Cfg is the capacity of dielectric film between floating        gates under an adjacent word line. The value C2/(C1+C2+Cag+Cfg)        is called a coupling ratio. To ensure that a voltage applied to        the control gate is efficiently transmitted to the floating gate        for the purpose of performing a programming/erasing operation,        it is essential that the coupling ratio be increased. To achieve        this purpose, it is important to (1) increase the capacity of        interpoly dielectrics C2, (2) reduce Cag by increasing the        thickness of film between the third gate and floating gate,        and (3) provide the floating gate with a U- or fin-shaped cross        section in order to reduce the cross-sectional area of the        floating gate and decrease the capacity of dielectric film Cfg        between opposing floating gates. To increase C2, it is necessary        to increase the floating gate surface area. In the above        publicly known example, however, it is difficult to decrease the        operating voltage because the floating gate 204 has a small        surface area. This problem is particularly crucial for an        erasing operation that is performed by applying a high voltage        to the dielectric film 210 between the floating gate and Si        substrate and emitting electrons accumulated in the floating        gate toward the substrate due to tunneling. For a multilevel        flash memory, which stores two bits of data per memory cell, the        memory cell threshold voltage difference between the programming        state and erasing state needs to be great. It is essential that        the coupling ratio be improved to reduce the programming/erasing        time.

The second task is to reduce deviation programming speed distributionand miniaturize the third gate. Various methods for forming a virtualgrounding type memory cell with a three-layer polysilicon gate arestated in JP-A No. 085541/2001. In a certain method stated in JP-A No.085541/2001, which forms a floating gate 204 after forming a third gate206, a gate bird's beak is generated. More specifically, the lower endof the third gate 206, which is formed earlier, is oxidized when atunnel dielectric film 210 is formed by means of thermal oxidation, sothat the gate oxide film thickness increases.

The reason is that the gate oxide film for the lower end of the thirdgate is removed in a cleaning process for tunnel dielectric filmformation. As a result, the lower end of the third gate polysilicon filmis oxidized. The growth of the gate bird's beak varies the thresholdvoltage of a MOS transistor, which is formed by the third gate, so thatdeviation of programming distribution of memory cell increase. Theincrease in the memory cell programming speed deviation causes anincrease in the number of verifications for judging whether a specifiedthreshold level is reached by a multilevel flash memory. This results inan increase in the chip programming time. Further, when the gate oxidefilm thickness of the third gate increases due to the growth of the gatebird's beak, the punch-through immunity of the MOS transistor formed bythe third gate decreases, making it difficult to reduce the gate length.

The third task is to miniaturize the word line. For high-density flashmemories, the word line is generally patterned to the minimum dimensionsfor memory cell miniaturization. To achieve this purpose, it isnecessary to provide an adequate focus margin in a lithography processfor word line patterning. Consequently, the third task is to minimizethe step coverage.

As described above, the development of a new nonvolatile semiconductormemory device and its manufacturing method has been called for in orderto miniaturize a virtual grounding type memory cell based on athree-layer polysilicon gate and accomplish the tasks for performanceenhancement.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a new nonvolatilesemiconductor memory device and its manufacturing method for the purposeof miniaturizing a virtual grounding type memory cell based on athree-layer polysilicon gate, enhancing the performance, and improvingthe yield.

A typical example of the present invention will now be brieflydescribed.

A nonvolatile semiconductor memory device according to the presentinvention comprises a memory cell, which includes a first conductivewell, which is formed in a silicon substrate; a second conductivesource/drain region, which is formed in the well; a channel formedperpendicularly to the source/drain region; a floating gate, which isthe first gate formed on the silicon substrate via a dielectric film; acontrol gate, which is the second gate formed above the floating gatevia dielectric film; a word line formed with the control gate connected;and a third gate, which is formed above silicon substrate, besidefloating gate, and below control gate via a dielectric film andexercises functions different from those of the floating gate andcontrol gate. To achieve the above object, the floating gate's two endsthat are positioned perpendicularly to the word line are placed over thetop of the third gate via the dielectric film.

The above floating gate is positioned in a gap in the third gate in sucha manner that the gap is not completely filled.

When the sidewall within the third gate space is A, the bottom withinthe third gate space is B, the flat section on the top of the third gateis C, and the sidewall on the top of the third gate is D, the surfacearea of the above floating gate is as follows:A>B+C+D

The above third gate provides split channel control gate. Alternatively,the above third gate exercises the functions of both an erase gate andsplit channel control gate. It is preferable that the dielectric filmbetween the above third gate and well should be identical with the gatedielectric film for a peripheral circuit low-voltage section. It is alsopreferable that the components and film thickness of the above thirdgate should be the same as those of a peripheral circuit gate.

In a nonvolatile semiconductor memory device comprising a memory cell,which includes a first conductive well formed in a silicon substrate; asource/drain region formed in the well; a channel formed perpendicularlyto source/drain region; a floating gate, which is the first gate formedabove the silicon substrate via a dielectric film; a control gate, whichis the second gate formed above the floating gate via dielectric film; aword line formed with the control gate connected; and a third gate,which is formed above silicon substrate, beside floating gate, and belowcontrol gate via a dielectric film=and exercises functions differentfrom those of the floating gate and control gate, the above object isachieved by positioning a contact hole for connecting the word line andmetal wiring on a member having the same material and film thickness asthe third gate via a dielectric film. In the above instance, it ispreferable that the above member should be a polysilicon film.

Further, in a nonvolatile semiconductor memory device comprising amemory cell, which includes a first conductive well formed in a siliconsubstrate; source/drain region formed in the well; a channel formedperpendicularly to the source/drain region; a floating gate, which isthe first gate formed above the silicon substrate via a dielectric film;a control gate, which is the second gate formed above the floating gatevia dielectric film; a word line formed with the control gate connected;and a third gate, which is formed above silicon substrate, besidefloating gate, and below control gate via a dielectric film andexercises functions different from those of the floating gate andcontrol gate, the above object is achieved by forming a secondconductive impurity region within a silicon substrate under a pluralityof third gate binds. In the above instance, the above source/drainregion, the second conductive impurity region, and a diffusion layerregion for a select transistor for selecting the second conductive thesource/drain region are connected.

Furthermore, the above object is achieved when a method formanufacturing a nonvolatile semiconductor memory device comprising amemory cell, which includes a first conductive well formed in a siliconsubstrate; a source/drain region formed in the well; a channel formedperpendicularly to the source/drain region; a floating gate, which isthe first gate formed above the silicon substrate via a dielectric film;a control gate, which is the second gate formed above the floating gatevia dielectric film; a word line formed with the control gate connected;and a third gate, which is formed above silicon substrate, besidefloating gate, and below control gate via a dielectric film andexercises functions different from those of the floating gate andcontrol gate comprises the steps of forming a plurality of wells on asemiconductor substrate; forming on the wells a first gate dielectricfilm and a second gate dielectric film, which differ in film thickness;forming a first polysilicon film on the first and second gate dielectricfilms; patterning the first polysilicon film to form a line and space ina first direction; forming a third gate dielectric film in the space;forming a second polysilicon film; patterning the second polysiliconfilm in the first direction; forming interpoly dielectrics; forming athird polysilicon film; patterning the third polysilicon film and secondpolysilicon film in a second direction which is perpendicular to thefirst direction; and patterning the first polysilicon film for a secondtime.

In the above instance, the step of patterning the first polysilicon filmto form a line and space in the first direction is performed solely by amemory cell array section. In this instance, it is preferable that thefirst gate dielectric film should be thicker than the third gatedielectric film.

Further, subsequently to line-and-space formation in the first directionby the above first polysilicon film, it is preferable that a third gatedielectric film should be formed after forming a dielectric filmsidewall on the polysilicon film.

Furthermore, the above first polysilicon film is patterned in such amanner that a line section is bound at an end of the line and space inthe first direction. In this instance, a second conductive impurity isintroduced under a bind for the above first polysilicon film patternbefore patterning. The patterning in the first direction for the abovesecond polysilicon film is performed on the first polysilicon filmpattern line. Alternatively, the patterning in the first direction forthe above second polysilicon film may be performed in such a manner asto embed the second polysilicon film into the space for the firstpolysilicon film pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram that illustrates a flash memory cell arrayaccording to a first embodiment of the present invention.

FIG. 2 is a plan view that illustrates the main parts of a flash memoryaccording to the first embodiment of the present invention.

FIGS. 3( a) to 3(d) are cross-sectional views that illustrate the mainparts of the flash memory.

FIGS. 4( a) to 4(f) are cross-sectional views that illustrate a methodfor manufacturing the flash memory.

FIGS. 5( a) to 5(f) are cross-sectional views that illustrate a methodfor manufacturing the flash memory.

FIGS. 6( a) to 6(e) are cross-sectional views that illustrate a methodfor manufacturing the flash memory.

FIG. 7 is a circuit diagram that illustrates a programming operation ofthe flash memory.

FIG. 8 is a circuit diagram that illustrates an erasing operation of theflash memory.

FIG. 9 is a circuit diagram that illustrates a read operation of theflash memory.

FIG. 10 illustrates threshold voltage changes that occur when the flashmemory performs a programming/erasing operation.

FIGS. 11( a) and 11(b) are cross-sectional views that illustrate thefinish of the flash memory cell.

FIGS. 12( a) to 12(d) are cross-sectional views that illustrate a methodfor manufacturing a flash memory according to a second embodiment of thepresent invention.

FIGS. 13( a) to 13(d) are cross-sectional views that illustrate a methodfor manufacturing a flash memory according to a third embodiment of thepresent invention.

FIG. 14 shows a cross section of a flash memory cell according to thepresent invention.

FIG. 15 shows a cross section of the major parts of a prior art flashmemory.

FIGS. 16( a) to 16(f) are cross-sectional views (section E-E′ of FIG. 2)that illustrate a method for manufacturing a memory cell and selecttransistor of a flash memory according to the present invention.

FIGS. 17( a) to 17(f) are cross-sectional views (section E-E′ of FIG. 2)that illustrate a method for manufacturing a memory cell and selecttransistor of a flash memory according to the present invention.

FIGS. 18( a) to 18(e) are cross-sectional views (section E-E′ of FIG. 2)that illustrate a method for manufacturing a memory cell and selecttransistor of a flash memory according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described indetail below.

First Embodiment

A first embodiment of the present invention will now be described withreference to FIGS. 1 to 6. FIG. 1 is a circuit diagram that illustratesa memory array in which flash memory cells are arranged in matrix form.FIG. 2 is a plan view of a memory cell array. FIGS. 3( a) to 3(d) arecross-sectional views of sections A-A′, B-B′, C-C′, and D-D′ of FIG. 2,respectively. FIGS. 4 to 6 are cross-sectional views that illustrate aprocess for memory cell/peripheral circuit manufacturing. FIGS. 16 to 18illustrate a manufacturing process for cross section E-E′.

In FIG. 1, GDL denotes a global data line whereas LDL denotes a localdata line. The memory cell array shown in this figure has a hierarchicaldata line structure. WL denotes a word line. AG denotes a third gate(assist gate). ST denotes gate wiring for a select transistor. SLdenotes a common source line.

As shown in FIGS. 3 and 6( e), each memory cell comprises a source/draindiffusion layer 113, which is within a p-type well 104 formed on asilicon substrate 101; a floating gate 115 b, which serves as a firstgate; a control gate 117 a, which serves as a second gate; and a thirdgate 109 a. The control gates 117 a of memory cells are connected in therow direction to form a word line. The floating gate 115 b and well 103are separated by gate dielectric film 114. The third gate 109 a and well103 are separated by gate dielectric film 108. The floating gate 115 band third gate 109 a are separated by dielectric film 114 a. Thefloating gate 115 b and word line 117 a are separated by dielectric film116 a. The third gate 109 a and word line 117 a are separated bydielectric film 110 a. The source/drain diffusion layer 113 ispositioned perpendicularly to the word line 117 a and exists as a localsource line and local data line for connecting the sources and drains ofmemory cells arranged in the column direction. In other words, thenonvolatile semiconductor memory device comprises a contactless array inwhich each memory cell has no contact hole. Channel formation takesplace in a direction perpendicular to the diffusion layer 113.

Two end faces of the third gate 109 a respectively face two end faces ofthe above floating gate 115 b, which are perpendicular to the word line117 a and channel, respectively, via dielectric film 114 a. The floatinggate 115 b is positioned in a gap in the third gate 109 a, which ispositioned perpendicular to the word line 117 a and channel. Further,the floating gate 115 b is symmetrical with respect to the third gate109 a, and the third gate 109 a is symmetrical with respect to thefloating gate 115 b.

In the present embodiment, a pair of diffusion layers 113, which form asource/drain, is positioned asymmetrically with respect to the patternof the floating gate 115 b, and an offset structure is employed so thatone diffusion layer does not overlap with the floating gate. The thirdgate 109 a and diffusion layer 113 exist so that they partly overlapwith each other.

A programming operation, erasing operation, and read operation will nowbe described with reference to FIGS. 7 to 9.

TABLE 1 Programming Erasing Read WL_(m) 13.5 V −18 V Vr WL_(m+1) 0 V 0 V0 V GDL_(m) 4.5 V 0 V 1 V GDL_(m+1) 4.5 V 0 V 1 V LDL_(mL) 0 V 0 V 0 VLDL_(mR) 4.5 V 0 V 1 V LDL_(m+1L) 0 V 0 V 0 V LDL_(m+1R) 4.5 V 0 V 1 VLDL_(m+2L) 0 V 0 V 0 V AG_(e) 0 V 0 V 0 V AG_(o) 1.1 V 0 V 3.5 V ST₁ 6 V0 V 6 V ST₂ 0 V 0 V 0 V ST₃ 0 V 0 V 0 V ST₄ 6 V 0 V 6 V

First, when select cell PCS1 shown in FIG. 7 is to be programmed, ahigh, positive voltage of, for instance, 13.5 V or so is applied to wordline WL_(m) and a low voltage of approximately 1.1 V is applied to thirdgate AG_(o). Further, a voltage of approximately 4.5 V is applied toglobal data line GDL_(m) and red to local data line LDL_(mF) via selecttransistor ST₁. Source LDL_(m+1L) and p well are maintained at 0 V.Channel formation then takes place in a well under third gate 109 a sothat a channel at an end of a floating gate on the source side generateshot electrons. The electrons are then injected into the floating gate.In other words, third gate 109 a functions as a gate that controls achannel existing underneath. The memory cells are such that theefficiency of hot electron generation and injection is higher than thatof a prior art NOR flash memory. The memory cells make it possible toperform programming in a region where the channel current is small.Therefore, an internal power supply having the same current supplycapacity as a flash memory chip that performs a programming operationbased on tunneling can perform a parallel programming operation for alarge number of memory cells on the order of kilobytes, therebyimproving the programming throughput.

For an erasing operation, a high, negative voltage of, for instance, −18V is applied to word line WL_(m) as shown in FIG. 8. In this instance,third gate AG_(e) and AG_(o), all source/drain diffusion layers DL, andwell are maintained at 0 V. Alternatively, a high, negative voltage of,for instance, −16 V is applied to word line WL_(m), a positive voltageof, for instance, 2 V is applied to the well, and third gate AG_(e) andAG_(o) and all source/drain diffusion layers DL are maintained at 0 V.This invokes a Fowler-Nordheim tunnel current flow from the floatinggate to the well, thereby allowing electrons accumulated on the floatinggate to be emitted.

When information is to be read from cell RSC1 shown in FIG. 9, a voltageis applied to word line WL_(m) in accordance with a multivaluedthreshold voltage level, and a voltage of approximately 3.5 V is appliedto third gate AG_(o). Further, a voltage of approximately 1 V is appliedto global data line GDL_(m) and fed to local data line LDL_(mR) viaselect transistor ST₁. Source LDL_(m+1L) and p well are maintained at 0V.

As indicated in 3(a), the present embodiment differs from the prior artin that two end faces of the floating gate 115 b that are perpendicularto the word line 117 a and channel are respectively placed over the topof the third gate 109 a via dielectric film 110 a. The film thickness ofthe floating gate 115 b is set so as not to completely fill the thirdgate space. The use of such a fin-shaped floating gate can reduce thecross-sectional area parallel to a word line, decrease the size ofdielectric film between opposing floating gates between adjacent wordlines, and increase the floating gate surface area. This ensures thatthe memory cell's coupling ratio improves to reduce the internaloperating voltage for use in a programming/erasing operation even if theword line pitch decreases due to miniaturization, thereby reducing thedistance between word lines. Further, since the dielectric film betweenopposing floating gates between adjacent word lines is small in size, itis possible to reduce the read threshold voltage discrepancy that arisesdepending on whether the adjacent bit threshold voltage is in aprogramming state or erasing state. Consequently, a multilevel memory,which stores at least two bits of data per memory cell with thethreshold voltage set for four or more levels, can compress thedistribution of each threshold voltage, thereby making it possible toreduce the amount of threshold voltage changes for a programming/erasingoperation. As a result, the reducing the programming/erasing time,conducting a low-voltage operation, and data retention characteristicsby alleviating an abandoned electric field can be improved.

In this instance, when the surface area of the floating gate 115 bwithin the sidewall of the third gate space is A, the surface area ofthe bottom section within the third gate space is B, the surface area ofthe flat top of the third gate is c, and the surface area of thesidewall on the top of the third gate is D as shown in FIG. 14, thefollowing equation holds:A>B+C+D  Equation (2)

For memory cell miniaturization, the third gate line and space needs tobe reduced. To increase the floating gate surface area under theseconditions, it is necessary to ensure that the values B and C are small,and that the value A or D is great. An increase in the value D causes anincrease in the capacity of the dielectric film between opposingfloating gates between adjacent word lines. Therefore, the arearelationship indicated by Equation (2) in which the sidewall area Awithin the third gate space is enlarged with the areas of the othersections minimized is effective for memory cell miniaturization,operating speed increase, and data storage characteristic improvement.

After third gate formation, a dielectric film sidewall is formed on thethird gate before the formation of floating gate dielectric film(so-called tunnel dielectric film). This makes it possible to restrainthe growth of the gate bird's beak at a lower end of the third gate 109a and miniaturize the gate length of the third gate. In addition, it isalso possible to reduce the threshold voltage variations of a MOStransistor formed by the third gate and restrain deviation ofprogramming speed distribution among memory cells. This results in adecrease in the number of verifications at the time of chip programmingand an increase in the programming throughput.

Further, the space formed at the time of third gate patterning ispositioned within a memory mat in which a floating gate is positioned.Therefore, the step on which word line 117 a is formed decreases. Thelithography focus margin for word line patterning then improves, therebymaking it possible to reduce the word line pitch.

A method for manufacturing the memory cells will now be described withreference to FIGS. 4 to 6 and FIGS. 16 to 18.

The nonvolatile semiconductor memory device comprises a memory cellregion and a peripheral circuit region. In the memory cell region, aplurality of memory cells for information storage are arranged in matrixform. In the peripheral circuit region, a plurality of MOS transistorsare mounted to form a peripheral circuit that provides necessary voltagegeneration within the chip. The peripheral circuit region can be dividedinto a low-voltage section to which only a voltage of 3.3 V or otherrelatively low voltage is applied, and a high-voltage section to which avoltage of 18 V or other high voltage required for reprogramming isapplied. As shown in 6(e), the low-voltage section and high-voltagesection both comprise a plurality of NMOS transistors and PMOStransistors, which are formed in P wells 104 b and 104 c and N wells 105a and 105 b. Memory cells are formed in P well 104 a. FIGS. 4 to 6 showa cross section that is parallel to memory cell word lines andperpendicular to peripheral circuit MOS transistor gate lines.

The manufacturing method will now be described.

First of all, a shallow groove isolation region 102 for isolating theselect transistor and peripheral circuit MOS transistor is formed on ap-type Si substrate 101 having a plane orientation (100). P well regions104 a, 104 b, and 104 c, N well regions 105 a and 105 b, and anisolation region 103 are formed by an ion implantation method (FIGS. 4(a) and 16(a)). Next, channel ion implantation (not shown) is performedto adjust the threshold voltages for the memory cell, select transistorand peripheral circuit MOS transistor. Then, ion implantation isconducted to form a diffusion layer (124 in FIG. 2 and FIG. 16( a-2))under a third gate bind (125 in FIG. 2). Thanks to the above ionimplantation, the memory cell diffusion layer wiring 113 and selecttransistor diffusion layer 120 a can be electrically connected (FIGS. 16to 18). Next, an approximately 23 nm silicon oxide film 106, whichserves as a high-voltage section gate dielectric film within theperipheral circuit region, is formed by thermal oxidation (FIG. 4( b)and FIG. 16( b)). Subsequently, a photoresist pattern is formed and thena wet etching method is applied to leave the silicon oxide film 106 inthe high-voltage section of the peripheral circuit region (the siliconoxide film 106 is then designated by the reference numeral 106 a) (FIG.4( c) and FIG. 16( c)). Next, the photoresist pattern is removed, andthen a 9 nm thermally oxidized film 108, which serves as a gatedielectric film for the select transistor and peripheral MOS transistoror a dielectric film for isolating the memory cell's third gate from thewell, is formed by thermal oxidation in the low-voltage section of theperipheral circuit region and in the memory cell region. In thisinstance, the resulting thermally oxidized film thickness for thehigh-voltage section of the peripheral circuit region is 25 nm (thesilicon oxide film 106 a is then designated by the reference numeral 106b) (FIG. 4( d) and FIG. 16( d)). Subsequently, a polysilicon film 109and a silicon oxide film 110, which serve as the electrodes for theselect transistor and peripheral MOS transistor or memory cell thirdgate, are sequentially deposited (FIG. 4( e) and FIG. 16( e)). Alithography and dry etching technology are then used to pattern siliconoxide films 110 and polysilicon films 109 (the silicon oxide films 110and polysilicon films 109 are then designated by the reference numerals110 a, 110 b and 109 a, 109 b, respectively). The employed patternarrangement scheme is such that the silicon oxide films 110 andpolysilicon films 109 in all non-memory-cell regions remain withoutbeing etched. Further, all the spaces formed by the patterning processare of the same dimensions to ensure that the word line polycide to beformed in a later process is uniformly embedded in the third gate spacewithin the chip and in flat stepped form (FIG. 4( f) and FIG. 16( f)).

Next, a silicon oxide film 111 is deposited d by low pressure CVD(Chemical Vapor Deposition) (FIG. 5( a) and FIG. 17( a)). This siliconoxide film 111 is then subjected to anisotropic etching so as to leaveonly the sidewall portion for the third gate pattern 109 (the siliconoxide film is then designated by the reference numeral 111 a) (FIG. 5(b) and FIG. 17( b)). The film provides protection to prevent the thirdgate oxide film from receding in a cleaning process before tunneldielectric film formation, thereby letting the gate bird's beak grow toincrease programming variations among cells and degrade the shortchannel characteristic of the third gate MOS. The thickness of thesilicon oxide film 111 is set so that the film is completely removed ina cleaning process performed immediately before tunnel dielectric filmformation but the amount of overetching is extremely small.Subsequently, arsenic tilt ion implantation and boron tilt ionimplantation are performed in different directions to form a memory cellsource/drain region 113 and punch-through stopper layer 112 (FIG. 5(c)). The diffusion layer 124 under the third gate is now connected tothe memory cell source/drain region 113 (FIG. 17( c)). Next, thedielectric film 114 for isolating the floating gate from the well andthe floating gate from the third gate is formed by thermal oxidation.The oxide film over the well is set to a thickness of 9 nm. In thisinstance, an approximately 20 nm oxide film 114 a grows on the thirdgate sidewall (FIG. 5( d) and FIG. 17( d)). A polysilicon film 115,which serves as a floating gate, is then deposited d in such a manner asto incompletely fill the third gate space (FIG. 5( e) and FIG. 17( e)).A lithography and dry etching technology are then used to pattern thedeposited d film in a direction parallel to the third gate (thepolysilicon 115 is then designated by the reference numeral 115 a). Theemployed structure is such that an end of the floating gate pattern 115a is placed over the third gate 109 a via the silicon oxide film 110 a(FIG. 5( f)).

Subsequently, a film stack of a silicon oxide film/silicon nitridefilm/silicon oxide film for isolating a floating gate from a word line,that is, an ONO film 116, a stack film of a polysilicon film and atungsten silicide film serving as a word line, that is, a polycide 117,and a silicon oxide film 118 are sequentially deposited. In thisinstance, the thickness of the polysilicon film serving as a lower layerfor the polycide 117 is adjusted so as to completely fill the memorycell space formed as indicated in FIG. 4( f) and make the surface of thepolycide 117 nearly flat (FIG. 6( a) and FIG. 18( a)). Next, a publiclyknown lithography and dry etching technology are used to pattern thesilicon oxide film 118 and polycide 117 to the minimum dimensions toaccomplish word line formation (the silicon oxide film 118 and polycide117 are then designated by the reference numerals 118 a and 117 a,respectively). Further, the word line 117 a is used as a mask to processthe ONO film 116 and polysilicon film pattern 116 a and finish thefloating gate (the ONO film 116 and polysilicon film pattern 115 a arethen designated by the reference numerals 116 a and 115 b, respectively)(FIG. 6( b) and FIG. 18( b)). A lithography and drying etchingtechnology are then used to pattern the silicon oxide film 110 b andpolysilicon film 109 b in the peripheral circuit and form a gateelectrode for the peripheral circuit MOS transistor (the silicon oxidefilm 110 b and polysilicon film 109 b are then designated by thereference numerals 110 c and 109 c, respectively) (FIG. 6( c)). Theabove process also forms a gate for the select transistor (127 in FIG. 2and FIG. 18( c)). Further, at an end of the memory mat, the polysiliconfilm 109 b and silicon oxide film 1110 b are patterned outside the wordline 117 a as shown in FIG. 2. Next, the low-concentration source/drainregions 119 a, 119 b, 120 a, 120 b are formed for the select transistorand peripheral circuit MOS transistor by an ion implantation method(FIG. 6( d) and FIG. 18( e)), a sidewall 121 for the silicon oxide filmis formed, and then the high-concentration source/drain regions 122 a,122 b, 123 a, 123 b are formed for the peripheral circuit MOS transistor(FIG. 6( e)). This connects the diffusion layer 124 under the thirdgate, the source/drain region 113 for the memory cell, and the diffusionlayer 120 a, 123 a for the select transistor so that the memory cellsource/drain is connected to the diffusion layer for the selecttransistor (FIG. 18( e)). Subsequently, although not shown, aninterlayer dielectric film is deposited and then contact hole (128 inFIG. 2) routed to a word line 117 a, a gate electrode 127, 109 c for theselect transistor and peripheral MOS transistor, and the source/drainregion 119 a, 119 b, 120 a, 120 b, 122 a, 122 b, 123 a, 123 b. A metalfilm is then deposited and processed to provide first layer metal wiring(129 in FIG. 2). Further, an interlayer dielectric film is formed andprovided with a through-hole (130 in FIG. 2), and then second layermetal wiring (131 in FIG. 2) is formed to mainly provide a global bitline. In addition, an interlayer dielectric film is deposited andprovided with a hole to form third layer metal wiring and then apassivation film to is formed to finish the nonvolatile semiconductormemory device.

FIG. 10 shows the programming/erasing characteristic of memory cellsthat are formed by the present invention. For comparison purposes, thisfigure also shows the characteristic of memory cells that are formedaccording to a method disclosed by Japanese Patent Laid-open No.2001-85541. Fin-shaped floating gates are employed to decrease thecross-sectional area and reduce the capacity of the dielectric filmbetween opposing floating gates. Further, the end of a floating gate isextended to the top of the third gate. Consequently, its surface area isincreased to increase the coupling ratio from 0.52 to 0.60. As a result,the programming/erasing speed is increased even when the same voltage isused for programming/erasing.

The result obtained by measuring the threshold voltage distributionafter programming for memory cells formed by the above method indicatesthat the threshold voltage difference between the fastest bit and theslowest bit is 2.1 V. On the other hand, the programming thresholdvoltage distribution exhibited by the prior art that does not form asidewall on the third gate is 4.7 V, which means that a great variationis involved. Further, the result obtained by measuring the cut-offcharacteristic under programming operating conditions for the split gateMOS transistor, which is composed by the third gate, indicates that thepresent invention can achieve cut-off even when the third gate length is0.20 μm whereas the prior art allows a punch-through to occur when thethird gate length is 0.25 μm and cannot readily accomplish cut-off. TheSEM (Scanning Electron Microscope) observation of the cross-sectionalshapes of the present invention and prior art, which has been made toclarify the difference in programming variations and cut-offcharacteristic between the present invention and prior art, has revealedthat the prior art allows a gate bird's beak to grow at a lower end ofthe third gate as shown in FIG. 11( a), whereas the present inventionrestrains the growth of the gate bird's beak as shown in FIG. 11( b).

Further, the end of a memory cell array formed by the present inventionhas a significantly smaller number of word line open circuits/shortcircuits than the counterpart formed by the prior art. It means that thepresent invention increases the yield. The reason is that the thirdgate, which provides a base for a word line, is subjected toline-and-space patterning in the memory cell array section only. As aresult, the contact hole region for connecting the word line to themetal wiring is positioned over the polysilicon film 109 b and thesilicon oxide film 110 b deposited on the polysilicon film 109 b so thatthe memory cell region is flush with the word line surface in the memorycell region. This increases the lithography focus margin for the wordline to be patterned to the minimum dimensions, thereby enhancing theyield.

The first embodiment, which has been described above, provides anadvantage of increasing the programming/erasing speed of a nonvolatilesemiconductor memory device. Further, it provides an advantage ofreducing the memory cell area. In addition, it also provides anadvantage of increasing the yield.

Second Embodiment

A second embodiment of the present invention will now be described withreference to FIG. 12. The second embodiment differs from the firstembodiment in that the formation of a third gate pattern 109 a, and theformation of a memory cell diffusion layer 113, and the formation of asidewall spacer 111 a that take place in that order. The secondembodiment's flash memory cell planar arrangement, finishedcross-sectional structure, and array structure are the same as those ofthe first embodiment and will not be described herein.

The memory cell manufacturing method according to the second embodimentwill now be described. First of all, a shallow groove isolation region102, P well regions 104 a, 104 b, 104 c, N well regions 105 a, 105 b, anisolation region between wells 103, gate dielectric films 106 a, 108,and a polysilicon film 109 a and a silicon oxide film 110 a, which serveas the electrodes for the memory cell third gate, are formed on asilicon substrate. As is the case with the first embodiment, theemployed pattern arrangement scheme is such that the silicon oxide films110 and polysilicon films 109 in all non-memory-cell regions remainwithout being etched. Further, all the spaces formed by the patterningprocess are of the same dimensions (FIG. 12( a)).

Next, arsenic tilt ion implantation and boron tilt ion implantation areperformed in different directions to form a memory cell source/drainregion 113 and punch-through stopper layer 112 (FIG. 12( b)). A siliconoxide film 111 is then deposited by low pressure CVD (Chemical VaporDeposition) (FIG. 12( c)). This silicon oxide film 111 is then subjectedto anisotropic etching so as to leave only the sidewall portion for thethird gate pattern 109 (the silicon oxide film is then designated by thereference numeral 111 a) (FIG. 12( d)). The film provides protection toprevent the third gate oxide film from receding in a cleaning processbefore tunnel dielectric film formation, thereby letting the gate bird'sbeak grow to increase programming variations among cells and degrade theshort channel characteristic of the third gate MOS. As is the case withthe first embodiment, the thickness of the silicon oxide film 111 is setso that the film is completely removed in a cleaning process performedimmediately before tunnel dielectric film formation but the amount ofoveretching is extremely small.

Subsequently, the processes to be performed after the formation of thegate dielectric film 114 are performed in the same manner as describedwith reference to FIG. 5( d) to FIG. 6( e), which illustrate the firstembodiment, to finish the memory cells (not shown).

As is the case with the first embodiment, the second embodiment of thepresent invention provides a higher programming/erasing speed than theprior art. It also reduces programming variations among memory cells andincreases the chip programming throughput. Further, it improves thecut-off characteristic of the split gate MOS transistor, which is formedby the third gate, and makes it possible to decrease the third gatelength. Furthermore, it enhances the yield.

Third Embodiment

A third embodiment of the present invention will now be described withreference to FIG. 13. The third embodiment differs from the secondembodiment in that a tunnel dielectric film is formed without completelyremoving, in a cleaning process, a silicon oxide film sidewall that isformed on the third gate pattern 109.

The memory cell manufacturing method will now be described. After thesame processing steps are performed to form a memory cell source/drainregion 113 and channel stopper layer 112 as indicated in FIG. 12( b) andpreceding figures illustrating the second embodiment (FIG. 13( a)), asilicon oxide film 111 is deposited by low pressure CVD (Chemical VaporDeposition) (FIG. 13( b)) and subjected to anisotropic etching so as toleave only the sidewall portion for the third gate pattern 109 (thesilicon oxide film is then designated by the reference numeral 111 a)(FIG. 13( c)). The silicon oxide film is thicker than the counterpartused with the first or second embodiment. Further, provision is made sothat the silicon oxide film is not removed in a cleaning process that isperformed immediately before tunnel dielectric film formation. Thesilicon oxide film 11 a not only provides protection at the time oftunnel dielectric film formation, as is the case with the first andsecond embodiments, to inhibit the gate bird's beak from growing toincrease programming variations among cells and the short channelcharacteristic of the third gate MOS from being degraded, but alsofunctions as a dielectric film that isolates the third gate 109 a fromthe floating gate 115 b.

After a tunnel dielectric film 114 and a polysilicon film 115, whichserves as a floating gate, are formed (FIG. 13( d)), the same processingsteps are performed to finish the memory cells (not shown) as indicatedin FIG. 5( f) and subsequent figures illustrating the first embodiment.

As is the case with the first and second embodiments, the thirdembodiment of the present invention provides a higherprogramming/erasing speed than the prior art. It also reducesprogramming variations among memory cells and increases the chipprogramming throughput. Further, it improves the cut-off characteristicof the split gate MOS transistor, which is formed by the third gate, andmakes it possible to decrease the third gate length. Furthermore, itenhances the yield.

As is the case with the second embodiment, the third embodiment forms asilicon film sidewall 11 a after a memory cell source/drain diffusionlayer 113 is formed. However, the same effect is produced even when thesilicon oxide film sidewall 111 a is formed before the formation of thesource/drain diffusion layer 113 as is the case with the firstembodiment.

In the above embodiment, a fin-shaped floating gate is employed andplaced over a third gate to increase the coupling ratio with a viewtoward programming/erasing characteristic improvement. However, even ifa floating gate is embedded between third gates as disclosed by JapanesePatent Laid-open No.2001-28428 and Japanese Patent Laid-openNo.2001-85541, a silicon oxide film sidewall can be formed on a thirdgate to restrain the growth of a bird's beak at a lower end of the thirdgate, reduce programming variations among memory cells, and enhance thechip programming throughput. Further, the cut-off characteristic of thesplit gate MOS transistor formed by the third gate can be improved toreduce the third gate length. Furthermore, when a flat pattern isemployed in a non-memory-cell region for a third gate on which a wordline is based, the step decreases, thereby increasing the lithographyfocus margin. This results in a remarkable decrease in the number ofword line open circuits/short circuits at an end of the mat, therebymaking it possible to increase the yield.

In the above embodiment, an erasing operation is performed by applying anegative bias to the word line, setting the other terminals to 0 V, andemitting electrons accumulated in the floating gate to the well.However, the same effect is produced even when an erasing operation isperformed by applying a negative bias to the word line and a positivebias to the third gate, setting the other terminals to 0 V, and emittingelectrons from the floating gate to the third gate.

In all the foregoing embodiments, at least two states are required forelectrons accumulated in a floating gate at the time of programming.However, the embodiments can be applied to multilevel storage in whichfour or more state levels are formed so that a single memory cell storestwo or more bits of data. In conventional multilevel storage, thedifference between the lowest threshold voltage state and the highestthreshold voltage state is greater than in two-level storage even whenthe amount of electrons accumulated in a floating gate is controlledwith high precision to compress the threshold voltage distribution ofeach level. Therefore, when Fowler-Nordheim programming is performed, aproblem arises so that the programming speed decreases or theprogramming voltage increases. The present invention, on the other hand,can reduce the voltage for programming/erasing to as low as 13 V or so.In other words, the present invention can raise the programming speedand is therefore extremely effective for multilevel storage.

While the present invention has been described in terms of the foregoingpreferred embodiments, it should be understood that the presentinvention is not limited to those preferred embodiments, and thatvariations may be made without departure from the scope and spirit ofthe present invention. For example, the present invention can be appliedto a one-chip microcomputer (semiconductor device) that comprises amemory cell array having a nonvolatile semiconductor memory device.

The representative embodiments of the present invention provide thefollowing advantages:

-   -   Reduces the memory cell area of a nonvolatile semiconductor        memory device.    -   Increases the operating speed of a nonvolatile semiconductor        memory device.    -   Enhances the yield of a nonvolatile semiconductor memory device.

1. A non-volatile semiconductor memory device comprising: a firstpattern portion provided above a semiconductor substrate as a chargestorage region; a second pattern portion provided above thesemiconductor substrate to one side of the first pattern portion; and athird pattern portion provided above the semiconductor substrate toanother side of the first pattern portion, wherein the first patternportion is formed so as to cover a lateral surface of the second patternportion and a lateral surface of the third pattern portion, wherein oneend of the first pattern portion is positioned on an upper surface ofthe second pattern portion, wherein another end of the first patternportion is positioned on an upper surface of the third pattern portion,wherein the first pattern portion is positioned in a gap region betweenthe second pattern portion and the third pattern portion, a part of thefirst pattern portion being concave, and wherein the first patternportion satisfies the relationship A>B+C+D, where a sidewall area of thefirst pattern portion within the gap region is A, a bottom surface areaof the first pattern portion within the gap region is B, an uppersurface area of a part of the first pattern portion over a top of secondpattern portion is C, and a side wall area of the part of the firstpattern portion over the top of the second pattern portion is D.
 2. Thenon-volatile semiconductor memory device according to claim 1, furthercomprising: a source region and a drain region disposed on a mainsurface of the semiconductor substrate; and a channel region formedbetween the source region and the drain region, wherein the firstpattern portion is formed above the channel region on a side toward thedrain region via a first gate dielectric film, wherein the secondpattern portion is formed above the channel region on a side toward thesource region via a second gate dielectric film, wherein a lateralsurface portion of the second pattern portion is covered with a firstdielectric film, and wherein the second pattern portion has a gateportion and a second dielectric film covering an upper surface of thegate portion.
 3. The non-volatile semiconductor memory device accordingto claim 2, wherein the gate portion controls the channel region on theside toward the source region via the second gate dielectric film. 4.The non-volatile semiconductor memory device according to claim 3,wherein the gate portion has a gate function as an erase gate.
 5. Thenon-volatile semiconductor memory device according to claim 2, whereinthe second gate dielectric film is formed from a same film as a gatedielectric film for a MOS transistor for a low-voltage section of aperipheral circuit formed on the semiconductor substrate.
 6. Thenon-volatile semiconductor memory device according to claim 2, wherein amaterial and a film thickness of the gate portion are the same as thoseof a gate for a MOS transistor for a peripheral circuit formed on thesemiconductor substrate.
 7. A non-volatile semiconductor memory device,comprising: a source region and a drain region disposed on a mainsurface of a semiconductor substrate; a channel region formed betweensaid source region and said drain region; a first gate provided abovesaid channel region on a side toward said drain region and via a firstgate dielectric film; a second gate provided above said channel regionon a side toward said source region via a second gate dielectric film; athird gate provided via a third dielectric film formed on said firstgate; a word line electrically connected to said third gate; a contacthole formed on said word line; and metal wiring that is connected tosaid word line via said contact hole, wherein said contact hole isprovided above a member having a same material and a same film thicknessas a film that forms said second gate.
 8. The non-volatile semiconductormemory device according to claim 7, wherein the member is a polysiliconfilm.
 9. The non-volatile semiconductor memory device according to claim7, wherein a lateral surface of said second gate is covered with a firstdielectric film, and an upper surface of the second gate is providedwith a second dielectric film.